In order to open the inventory of the supply truck. You must select "examine" option on the button right of your screen. This will open the supply truck inventory.

Your artillery cannon has run out of ammunition. You can get one of your crew members to transfer supplys from the supply van to the inventory of the crew member. The crew member then has to transfer that to the artillery cannon inventory.

Step 1 - Transfer Needed supplies to Solider's inventory.

Step 2 - Open inventory for artillery cannon along with solider's. Now transfer items.

Step 3 - This is what itll look like when transfer completed.

My hope is this guide helps clear some confusion on how you use this supply truck.